UV curing is based on photoinitiated polymerisation of functional oligomers and monomers into a crosslinked polymer network. When an ultraviolet curable coating is exposed to UV energy in this way a relatively hard film, having an extremely smooth surface, and hence one of high gloss, is produced. With the increasing popularity of radiation cured coatings for a wide variety of applications, the ability to control and reduce gloss is becoming more important. It is well known that matt surfaces provide the finished article with a more elegant appearance and hide imperfections at the surface, particularly in wood, furniture and PVC flooring applications, and several different methods of reducing the gloss of UV curable coatings have been reported, for example the use of "dual cure" or "gradient intensity cure" techniques, specific photoinitiators and non-silica type matting agents.
Traditional silica matting agents are conveniently used to reduce the gloss of solvent and water based finishes and in the UV industry synthetic silicas are used to provide a semi-gloss or matt effect, although as a rule high concentrations are generally required by the formulator. The high solids nature of UV systems and the absence of adequate film shrinkage, required to ensure optimal levels of particles are present in the surface of the cured film, makes efficient matting difficult. Accordingly, high concentrations of conventional silicas are required to achieve an acceptable degree of gloss reduction. Such high levels of silica can frequently cause changes in the rheological properties of the lacquer which can be detrimental to the coating and curing process and, can impair the optical properties of the cured film.
In an attempt to overcome this problem, the use of large particle size silicas has been promoted in the past for both thin and thick film applications. Such materials may produce an unacceptable level of roughness particularly in thin coatings and, depending on the viscosity of the system, can result in a greater tendency to settle on prolonged storage. It would be desirable therefore to provide a synthetic silica matting agent for UV systems, having good efficiency and minimal effect on formulation viscosity and film properties.
There is therefore a need for a new matting agent which overcomes these problems.